FR3116126A1 - LIDAR SYSTEM FOR DIFFENTIAL ABSORPTION AND BACKGROUND DISTANCE MEASUREMENTS - Google Patents

Abstract

A LIDAR system is suitable for performing differential absorption measurements of a chemical compound between two distinct optical frequencies (ν1, ν2), and measuring a distance away from an obstacle which is present in the background of a measurement zone where absorption occurs. A transmit optical power value is varied between different time intervals during a radiation transmit sequence, to enable realization of the LIDAR system by fiber optic technology, while having a power of sufficient emission. The LIDAR system makes it possible to evaluate a quantity of the chemical compound which is contained in the measurement zone, as well as a distance away from an obstacle which is located in the background of said measurement zone. : Figure 1

Description

LIDAR SYSTEM FOR DIFFENTIAL ABSORPTION AND BACKGROUND DISTANCE MEASUREMENTS

This description relates to a LIDAR system that is suitable for performing differential absorption and background distance measurements. It also relates to a method for measuring a quantity of a chemical compound, which uses such a system.

It is known to use differential absorption measurements to evaluate the quantity of a chemical compound which is contained in a measurement zone. For this, the absorption of radiation which is emitted in the direction of the measurement zone is measured for a first frequency of the radiation which does not correspond to an absorption band of the chemical compound, and also for a second frequency of the radiation which corresponds to an absorption band of the chemical compound. It is therefore necessary to emit radiation at both frequencies in the direction of the measurement area, and to compare the absorption levels that are measured for each of them. Such a measurement method is commonly referred to as IPDA, for “Integrated Path Differential-Absorption” in English, or integrated differential absorption on a path. It also requires knowing the depth of the measurement zone, such as, for example, the distance at which there is an obstacle which is present in the background of this measurement zone.

Performing such IPDA measurements using a LIDAR system is advantageous, in particular because of the detection sensitivity and the ability to analyze the detection signals provided by a system of this type. It is then an important issue to be able to use LIDAR systems which are made from optical fibers, because of the reduced size and weight of such systems, their better energy efficiency, and their robustness with respect to risks of loss of alignment between the optical components that make them up.

However, IPDA measurements require the following requirements to be met:
- at least two rays must be able to be emitted with different values of optical frequency, sufficiently quickly one after the other with respect to variations which may affect the content of the measurement zone. More precisely, pulses of the two radiations must be emitted with a repetition frequency which is sufficiently high with respect to the variations of the content of the measurement zone, and to allow the analysis of the detection signals, for example with a repetition frequency which is greater than 1 kHz (kilohertz);
- the two radiations, that outside the absorption bands of the chemical compound object of the measurement, and that which corresponds to one of its absorption bands, must be emitted with sufficient respective energies;
- that of the radiation which corresponds to one of the absorption bands of the chemical compound must have sufficient spectral fineness, in order to provide sufficient precision when determining the absorption by the chemical compound. In particular, it may be necessary for this spectral width of the radiation which corresponds to one of the absorption bands of the chemical compound to be less than 100 MHz (megahertz), when the central wavelength of this radiation is of the order of 1.6 µm (micrometer); And
- distance distance measurement for background obstacle, by characterization of the time of flight, requires the use of radiation pulses which are quite short, typically with individual durations which are less than about 100 ns (nanosecond ).

But the well-known phenomenon of stimulated Brillouin scattering that occurs in optical fibers limits the peak power of the radiation pulses that can be emitted by LIDAR systems made from optical fibers. This results in a limitation on the maximum reach distance of these fiber optic LIDAR systems.

Technical problem

Given these constraints, an object of the present invention is to propose a new LIDAR system which allows both to perform IPDA measurements and to determine the distance away from an obstacle which is present in the background of the measurement zone, and which can be made using optical fibres.

More particularly, the object of the invention is to propose such a LIDAR system which is produced by using at least one optical fiber to transmit the radiation emitted, but for which the power-peak limitation which is caused by the stimulated Brillouin scattering is pushed back. or avoided.

To achieve this goal or another, a first aspect of the invention proposes a LIDAR system which is adapted to carry out measurements of differential absorption between two distinct optical frequencies, and measurements of distance away from an obstacle which is present in the background of a measurement area where absorption occurs. This LIDAR system of the invention comprises:
- A laser source assembly, which is capable of producing radiation at either of the two optical frequencies;
- intensity modulation means, which are adapted to apply to each radiation a form of pulse envelope, including a pulse duration and a pulse optical power value; And
- a transmission controller, which is suitable for controlling the intensity modulation means.

In the context of the invention, by pulse optical power is meant a value which characterizes the intensity of each pulse, this value being able to correspond to a peak power of the pulse, or an average power which is evaluated over all the duration of the pulse, it being understood that the peak power value and the average power value increase as a function of the other at constant duration and shape of the pulse.

According to the invention, the LIDAR system is adapted so that, during operation of this LIDAR system, it emits in a target direction in which a measurement of differential absorption and a measurement of the distance away from the background obstacle are to be produced, a radiation emission sequence which includes:
- first time intervals during which the emission of radiation is located spectrally at a first of the two optical frequencies, with a first spectral width, a first pulse duration and a first value of optical power of the pulse; And
- second time intervals during which the radiation emission is spectrally located at the second of the two optical frequencies, with a second spectral width, a second pulse duration and a second pulse optical power value.

The first and second time slots can form any succession in the radiation emission sequence, with any number of first time slots between two second time slots, and vice versa. Alternatively or in combination, the radiation emission sequence may comprise any number of pulses located around the first optical frequency between two pulses located around the second optical frequency, and vice versa.

In addition, the radiation emission sequence has the following characteristics:
the first and second spectral widths are such that the emission of radiation during the first time intervals and during the second time intervals corresponds to spectral intervals which are disjoint, the first spectral width being greater than the second spectral width,
the first pulse optical power value is greater than the second pulse optical power value, and
the first pulse duration being shorter than the second pulse duration.

Such a LIDAR system can be made based on optical fibers, and in particular its laser source assembly can be of the MOPFA type, for "Master Oscillator Power Fiber Amplifier" or fiber power amplifier of a master oscillator. For this MOPFA type of LIDAR systems, pulses of radiation which have the desired spectral widths and are modulated according to the desired envelope shapes, and which are separated between successive pulses, are first produced, then these pulses are amplified before being transmitted. be emitted outwards.

Thanks to the first spectral width which is greater than the second spectral width, the first pulse optical power value can be chosen to be high or very high without the stimulated Brillouin scattering effect which would appear in optical fibers used to produce a such LIDAR system, does not interfere with the operation or use of this system.

For an IPDA measurement use, the radiation emission during the first time intervals is to be chosen outside the absorption bands of the chemical compound which is concerned by the measurements. This radiation emission during the first time intervals is further used to estimate the distance away from the obstacle which is present in the background of the measurement area. This telemetric measurement proceeds by determining the time of flight of the radiation pulses which are emitted during the first time intervals, for the round trip between the LIDAR system and the background obstacle. During these first time intervals, the high pulse optical power, allowed by the higher spectral width, and the shorter pulse duration allow a higher precision in the estimation of the distance from the obstacle to the obstacle. background. The emission of radiation during the second time intervals is to be chosen from one of the absorption bands of the chemical compound. During the second time intervals, the lower spectral width of the pulses provides greater precision in estimating the quantity of the chemical compound. The quantity of the chemical compound is estimated from the absorption ratio which is determined by the LIDAR system, between the radiation emitted during the second time intervals around the second optical frequency and those emitted during the first time intervals around the first optical frequency, and taking into account the estimated value for the distance away from the background obstacle.

Preferably, one or more of the following characteristics can be selected by the transmission controller:
- a repetition frequency of the radiation emission sequence can be between 1 kHz and 50 kHz;
- the first time intervals can have an individual duration of between 10 ns and 200 ns, preferably between 50 ns and 100 ns;
- the second time intervals can have an individual duration comprised between 0.1 μs (microsecond) and 10 μs, preferably between 0.5 μs and 5 μs;
- the first spectral width of the radiation which is emitted by the LIDAR system in the measurement zone, during the first time intervals, can be between 100 MHz and 2000 MHz, preferably between 500 MHz and 1000 MHz; And
- the second spectral width of the radiation which is emitted by the LIDAR system in the measurement zone, during the second time intervals, can be between 10 MHz and 200 MHz, preferably between 50 MHz and 100 MHz.

Finally, the laser source assembly can be adapted so that the radiation which is emitted by the LIDAR system in the measurement zone has first and second optical frequency values, respectively during the first and second time intervals, which correspond to wavelengths comprised between 1.3 μm and 1.8 μm, in particular between 1.5 μm and 1.6 μm, or which are situated around 2 μm. Such wavelength ranges are particularly suitable for measuring a quantity of carbon dioxide which is contained in the measurement zone.

Furthermore, the LIDAR system of the invention comprises a detection path which is suitable for independently detecting, processing and analyzing backscattered radiation which corresponds to the first optical frequency and to the second optical frequency, and which respectively correspond to emissions during the first and second time intervals.

Possibly, the LIDAR system may further comprise a calculation unit which is connected as input to at least one output of the detection channel. This calculation unit is adapted to provide estimates of the distance away from the background obstacle and of the quantity of the chemical compound which is contained in the measurement zone, from analysis signals which are produced by the detection pathway.

Depending on different embodiments which are possible for the invention, and in particular depending on different types of laser oscillators which are used in the laser source assembly to produce the radiation at each of the two optical frequencies, the first and/or or the second spectral width may be intrinsic or produced by dedicated spectral broadening means. By intrinsic spectral width is meant the spectral width of the radiation as produced by the corresponding laser oscillator. In other words, in the first case, the spectral width of the pulses which are emitted during the first and/or the second time intervals corresponds to the spectral width of the corresponding laser oscillator. Otherwise, the LIDAR system further includes:
- Spectral broadening means, which are arranged to modify the spectral width of at least one of the radiation produced by the laser source assembly.

In first embodiments of the invention, the emission controller can be adapted to control the laser source assembly, the intensity modulation means and, if applicable, and the spectral broadening means so that , in the radiation emission sequence, the first of the two optical frequencies is associated exclusively with the first spectral width, the first pulse duration and the first pulse optical power value, excluding the second spectral width, the second pulse duration and the second pulse optical power value, within first radiation pulses, and the second of the two optical frequencies is associated exclusively with the second spectral width, the second pulse duration and the second pulse optical power value, excluding the first spectral width, the first pulse duration and the first pulse optical power value, a u within second pulses of radiation which are separated from the first pulses.

For such first embodiments, the LIDAR system may have an arrangement in which the laser source assembly includes a first laser oscillator which is adapted to produce the radiation at the first optical frequency, and a second laser oscillator which is adapted to produce the radiation at the second optical frequency with the second spectral width. The spectral broadening means then comprise a phase modulator which is arranged on a path of the radiation produced by the first laser oscillator, and which is controlled by the emission controller to provide the first spectral width to this radiation produced by the first laser oscillator. In addition, the LIDAR system further comprises an optical switch which is controlled by the emission controller to transmit either the radiation coming from the phase modulator or the radiation produced by the second laser oscillator, to a downstream portion of the optical path of emission which is common to these radiations originating from the phase modulator and produced by the second laser oscillator, this downstream portion of the emission optical path comprising the intensity modulation means.

According to another arrangement which is also possible for the first embodiments of the invention, the laser source assembly likewise comprises a first laser oscillator which is adapted to produce the radiation at the first optical frequency, and a second laser oscillator which is adapted to produce the radiation at the second optical frequency with the second spectral width. The spectral broadening means further comprise a phase modulator which is arranged on a path of the radiation produced by the first laser oscillator, and which is controlled by the emission controller to provide the first spectral width to this radiation produced by the first laser oscillator. But in this other arrangement, the intensity modulation means comprise a first intensity modulator which is arranged on a path of the radiation coming from the phase modulator, and which is controlled by the emission controller to be effective on this radiation from the phase modulator. They further comprise a second intensity modulator which is arranged in a path of the radiation produced by the second laser oscillator, and which is controlled by the emission controller to be effective on this radiation produced by the second laser oscillator. Then, the LIDAR system further comprises an optical coupler adapted to transmit the radiation coming from the first and second intensity modulators to a downstream portion of the optical transmission path which is common to these radiation coming from the first and second intensity modulators .

For these two arrangements, the downstream portion of the transmit optical path may comprise an optical radiation amplifier, or an optical radiation amplifier chain, which is controlled by the transmit controller to produce the first and second power values optics in accordance with the characteristics of the invention for the radiation emission sequence.

In second embodiments of the invention, which are alternative with respect to the previous first embodiments, the emission controller can be adapted to control the laser source assembly, the spectral broadening means and the modulation means of intensity so that the radiation emission sequence comprises a succession of radiation pulses which are spectrally located either at the first of the two optical frequencies, or at the second of the two optical frequencies, all the radiation pulses having the same form of envelope which comprises, for the first optical frequency as for the second optical frequency, a first duration during which the emission of radiation has the first spectral width and the first value of optical power, and a second duration during which the the radiation emission has the second spectral width and the second optical power value, the first duration being greater than c shorter than the second duration, and being earlier or later than this second duration in each radiation pulse.

Generally for the invention, the LIDAR system can be made using fiber optic technology.

Still generally for the invention, the LIDAR system may comprise polarization means adapted so that the radiation which is emitted in the direction of the measurement zone by this LIDAR system has polarizations which are orthogonal, in particular circular polarizations which are opposite, when these radiations are emitted during the first time intervals or the second time intervals. In this case, the detection channel may comprise a polarization splitter which is arranged to transmit the backscattered radiation, depending on the polarizations of these backscattered radiation, either to a first detector which is sensitive in a first spectral interval including the first optical frequency combined with the first spectral width, or to a second detector which is sensitive in a second spectral range including the second optical frequency combined with the second spectral width. Possibly, the two detectors can be identical if their common sensitivity spectral range includes both the first optical frequency combined with the first spectral width and the second optical frequency combined with the second spectral width.

A second aspect of the invention proposes a method for measuring a quantity of a chemical compound which is present in a target direction, according to which:
- a LIDAR system which is in accordance with the first aspect of the invention, is selected such that the chemical compound has an absorptive power value which is lower at the first optical frequency than at the second optical frequency;
- the LIDAR system is oriented along the target direction to emit radiation according to the radiation emission sequence towards a measurement zone which is likely to contain the chemical compound, and an operation of the LIDAR system is triggered;
- the distance away from the obstacle which is present in the background of the measurement zone is estimated from the backscattered radiation which is relative to the first optical frequency and which corresponds to emissions during the first time intervals; And
- the quantity of the chemical compound which is contained in the measurement zone, integrated on the path of the pulses between the LIDAR system and the background obstacle, is estimated from intensity values which are separately related to the radiation backscattered at the first optical frequency and at the second optical frequency, corresponding respectively to the first and second time intervals in the radiation emission sequence, these backscattered radiations having been detected by the detection path of the LIDAR system.

Preferably, the distance away from the obstacle which is present in the background of the measurement zone can be estimated from a time of flight which is measured for the backscattered radiation relating to the first optical frequency. In this case, this remoteness distance, as estimated from the backscattered radiation which is relative to the first optical frequency, can be used to estimate the quantity of the chemical compound integrated on the path of the pulses which is contained in the zone measurement, in combination with the intensity values which relate separately to the radiation backscattered and detected at the first optical frequency and at the second optical frequency, and corresponding respectively to the emissions during the first and second time intervals.

The chemical compound which is concerned by the measurement method of the invention may be one of carbon dioxide, or CO ₂ , methane, or CH ₄ , nitrous oxide, or N ₂ O, and the water, or H ₂ O.

Finally, different conditions for implementing the invention may be as follows:
- For the first conditions of implementation, the LIDAR system can be installed on the surface of the Earth and directed to measure the quantity of the chemical compound which is present between this LIDAR system and the obstacle;
- for second conditions of implementation, the LIDAR system can be embarked on board an aircraft in flight and directed towards a geographical area on the surface of the Earth, to measure the distance away from the surface of the Earth in that geographic area relative to the LIDAR system, and to measure the amount of the chemical compound that is present between the LIDAR system and the Earth's surface in the same geographic area; And
- for third conditions of implementation, the LIDAR system can be embarked on board a satellite in orbit around the Earth and directed towards a geographical area on the surface of the Earth, to measure the distance away from the Earth's surface in that geographic area relative to the LIDAR system, and to measure the amount of the chemical compound that is present between the LIDAR system and the Earth's surface in the same geographic area.

For the second and third conditions of implementation, the surface of the Earth in the geographical area towards which the LIDAR system is directed, has the function of an obstacle present in the background of the measurement area.

Brief description of figures

The characteristics and advantages of the present invention will appear more clearly in the detailed description below of non-limiting exemplary embodiments, with reference to the appended figures, among which:

shows spectral variations of a first radiation emission sequence as implemented in first possible embodiments for the invention;

shows emission optical power variations for the first radiation emission sequence of ;

shows spectral variations of a second radiation emission sequence as implemented in second possible embodiments for the invention;

shows emission optical power variations for the second radiation emission sequence of ;

is a block diagram of a transmission path of a LIDAR system according to the invention, which is adapted to produce the first radiation emission sequence of And ;

is a block diagram of a first transmission path variant with respect to the system of , for another LIDAR system according to the invention, and again to produce the first radiation emission sequence of And ;

is a block diagram of a second transmission path variant with respect to the system of , for yet another LIDAR system according to the invention, and still for producing the first radiation emission sequence of And ;

is a block diagram of a third transmission path variant with respect to the system of , for yet another LIDAR system according to the invention, and still for producing the first radiation emission sequence of And ;

is a block diagram of a fourth transmission path variant with respect to the system of , for yet another LIDAR system according to the invention, and still for producing the first radiation emission sequence of And ;

is a block diagram of a transmission path of yet another LIDAR system according to the invention, and which is further adapted to produce the first radiation transmission sequence of And ;

is a block diagram of an alternative transmission path with respect to the system of , for yet another LIDAR system according to the invention, and which is further adapted to produce the first radiation emission sequence of And ;

is a block diagram of a transmission path of yet another LIDAR system according to the invention, which is adapted to produce the second radiation emission sequence of And ;

is a block diagram of an alternative transmission path with respect to the system of , for yet another LIDAR system according to the invention, and which is further adapted to produce the second radiation emission sequence of And ;

is a block diagram of a detection channel of a LIDAR system according to the invention;

is a block diagram of an alternative detection path with respect to the system of ;

is a block diagram of another variant of detection path with respect to the system of ; And

is a block diagram of yet another alternative detection pathway with respect to the system of .

Detailed description of the invention

For the sake of clarity, the dimensions of the elements which are represented in , , And correspond neither to actual dimensions nor to actual ratios of dimensions. Furthermore, all the elements are represented only symbolically in the figures, and identical references which are indicated in different figures designate elements which are identical or which have identical functions.

In the diagrams of , , And , the abscissa axis identifies the time, denoted t, during radiation emission sequences which consist of radiation pulses situated spectrally either around a first optical frequency, denoted ν ₁ , or around a second optical frequency, denoted ν ₂ . Δt ₁ and Δt ₂ respectively denote durations of first and second time intervals during these radiation emission sequences. In And , the ordinate axis identifies the values of the optical transmission frequency, denoted ν, for each instant of the transmission sequence concerned, and in And , the ordinate axis identifies the instantaneous emission power of the radiation, denoted P. In general, the radiation pulses which are designated by the reference 1 are located spectrally around the optical frequency ν ₁ , and assumed to be all identical between them. Similarly, the radiation pulses which are designated by the reference 2 are located spectrally around the optical frequency ν ₂ , and also assumed to be all identical to each other. For the application of the invention to measurements of differential absorption of a chemical compound between two distinct optical frequencies, all the radiation pulses 1 and 2 are emitted in the direction of a measurement zone which is likely to contain a quantity to be determined of the chemical compound. The optical emission frequency ν ₁ is intended to be selected outside the absorption bands of the chemical compound, and the optical emission frequency ν ₂ is intended to be selected within one of the absorption bands of this chemical compound. In the case where the chemical compound concerned is carbon dioxide, the optical emission frequency ν ₁ can be chosen equal to 190.81 THz, corresponding to a wavelength value λ ₁ equal to 1572.2 nm, and the emission optical frequency ν ₂ can be chosen equal to 190.84 THz, corresponding to a wavelength value λ ₂ equal to 1572.02 nm.

And relate to a same first radiation emission sequence which is possible to implement the invention. In such a first transmission sequence, all the radiation pulses 1 have first common values of spectral width Δν ₁ and of peak transmission power P ₁ , and have a pulse duration equal to Δt ₁ . Likewise, all the radiation pulses 2 of this same emission sequence have second common values of spectral width Δν ₂ and of peak emission power P ₂ , and have a pulse duration equal to Δt ₂ . The transmission peak power values P ₁ and P ₂ correspond to the radiation emitted by a LIDAR system in accordance with the invention, such that this transmission radiation leaves the LIDAR system in the direction of the measurement zone, in particular after the final optical amplification was performed inside the LIDAR system. For this first radiation emission sequence, the pulses 1, of individual durations Δt ₁ , correspond to the first time intervals as introduced in the general part of the present description, and the pulses 2, of individual durations Δt ₂ , correspond to the second time intervals. Thanks to the spectral broadening which is greater for pulses 1 compared to pulses 2, that is to say Δν ₁ >Δν ₂ , the peak transmission power value P ₁ can be greater than a limit of power due to stimulated Brillouin scattering, denoted P _SBS , which occurs in optical fibers of the LIDAR system, whereas the transmission peak power value P ₂ can be lower than this same power limit P _SBS .

And relate to the same second radiation emission sequence which is also possible to implement the invention. In such a second emission sequence, all the radiation pulses 1 and 2 which are emitted in the direction of the measurement zone likely to contain the quantity to be determined of the chemical compound, have the same form of envelope, which is transposed to the optical frequency of each pulse, the latter alternating between ν ₁ for pulses 1 and ν ₂ for pulses 2. This form of envelope comprises, inside each pulse 1, 2, a first time interval of duration Δt ₁ during which the radiation of pulse 1 or 2 has the spectral width value Δν ₁ and the peak emission power value P ₁ , and a second time interval of duration Δt ₂ during which the radiation of pulse 1 or 2 has the spectral width value Δν ₂ and the peak transmission power value P ₂ . The chronological order between the first and second time slots, each with its associated spectral width and peak transmit power values, may be reversed within each pulse, and the first and second time slots may optionally also be separated by an intermediate envelope-shaped pattern within each pulse. As before, the power limit P _SBS which is associated with a stimulated Brillouin scattering effect can be lower than the peak transmission power value P ₁ and higher than the peak transmission power value P ₂ .

For the first transmission sequence ( And ) as for the second transmission sequence ( And ), the radiation emissions at the optical frequency ν ₁ during the durations Δt ₁ are used to determine the distance away from the background obstacle. In the case of the second radiation emission sequence, the parts of the pulses 1 which correspond to the durations Δt ₂ , with the spectral width value Δν ₂ and the emission peak power value P ₂ , may not be used to estimate the distance away from the rear obstacle. However, the parts of the pulses 2 which correspond to the durations Δt ₁ , with the spectral width value Δν ₁ and the emission peak power value P ₁ , can optionally be used in addition to the parts of the pulses 1 which correspond to the durations Δt ₁ to determine the distance away from the background obstacle.

A generalization of the second sequence of pulses can be that each pulse 1 or 2, respectively around the optical frequency ν ₁ or ν ₂ , presents a steep edge of optical power rise, which is spectrally broad and which makes it possible to carry out range measurement, followed by an optical power decrease which is slow and spectrally fine, suitable for differential absorption measurement.

For these two radiation emission sequences, respectively according to And for the first, and according to And for the second, the following numerical values are given by way of non-limiting example:
the pulse repetition frequency can be between 1 kHz (kilohertz) and 50 kHz,
Δt ₁ can be between 50 ns and 100 ns,
Δt ₂ can be between 0.5 µs and 5 µs,
Δν ₁ can be between 500 MHz and 1000 MHz,
Δν ₂ can be between 50 MHz and 100 MHz,
P ₁ can be greater than 200 W, and
P ₂ can be greater than 50 W.
Thus, the duration Δt ₁ of the first time intervals can be shorter than the duration Δt ₂ of the second time intervals. In addition, the first spectral width value Δν ₁ may be greater than the second spectral width value Δν ₂ , and the first transmission peak power value P ₁ may be greater than the second peak power value of emission P ₂ . Then, thanks to the first value of spectral width Δν ₁ which is increased, the value P ₁ is distributed over an emission spectral interval which is wider than that over which the value P ₂ is distributed. For this reason, the value P ₁ may be greater than the stimulated Brillouin scattering threshold P _SBS which corresponds to the optical fibers used to produce the LIDAR system. Preferably, the value P ₂ can be chosen to be less than or equal to this stimulated Brillouin scattering threshold, in order to limit a loss of energy efficiency in the production of the radiation to be emitted during the second time intervals, of individual durations Δt ₂ .

We now describe several architectures of LIDAR systems which are in accordance with the invention, and which are designed to emit radiation sequences as described above. The description of these architectures is limited to the organization of their main components, it being understood that the person skilled in the art knows such components which are commercially available, and will know how to combine them according to the architectures described, without difficulty or need for inventive step. . Furthermore, it is understood that additional components to be used in these architectures, but which are not directly related to the principle of the invention, and whose use is common, are not described for the sake of clarity. All these LIDAR system architectures which are described below can advantageously be implemented using fiber optic technologies, or integrated optical circuit technologies, for the production of optical, electro-optical and interconnection components which are used. In those of the figures which show transmission channel architectures, the reference 50 designates a transmission controller, which is denoted CTRL and connected to the components of the transmission channel to produce the radiation transmission sequence which has the desired characteristics. The control mode to be implemented by this emission controller 50 is within the reach of those skilled in the art once the radiation emission sequence which is to be produced is provided to them.

shows a first possible emission path architecture for a LIDAR system according to the invention, which is designed to produce radiation emission sequences in accordance with And . The reference 10 designates a laser source, commonly called a laser oscillator, which is capable of producing a continuous laser beam at the optical frequency ν ₁ . This may for example be a laser diode or a fiber laser. The laser beam which is produced by the source 10 is sent through a phase modulator 11, denoted MOD. PHASE, which gives it the spectral width Δν ₁ . In this case, it is a phase modulation external to the laser source. The phase modulator 11 can be an electro-optical modulator, for example. A random generator of binary signals, as usually known by the acronym PRBS for "Pseudo-Random Binary Sequence" in English, or a radiofrequency noise generator, or even an electrical generator of arbitrary waveforms, known by the acronym AWG for "Arbitrary Waveform Generator" in English, can be connected to an electrical control input of the phase modulator 11. The one that is used among these alternative modes of control of the phase modulator 11 is designated by the reference 11c and denoted GENERATOR . In the case of use of a PRBS generator, the latter produces phase jumps which are equal to −π or π according to a random or pseudo-random sequence. The phase modulator 11 can then be advantageously associated at its output with an optical apodization filter (not shown) to suppress secondary lobes that can be generated by such a method of spectral broadening in the spectrum of the radiation which comes directly from the phase modulator 11. In the case of use of an AWG generator, the latter can be programmed to produce various waveforms such as a succession of ramps whose slopes are randomly variable between successive ramps. Alternatively, it can be programmed to produce an electrical control signal which is sinusoidal or which is a linear combination of several sinusoidal components. Other forms for the electrical control signals which are intended for the phase modulator 11 can also be used alternatively, it being understood that the person skilled in the art knows how to select the characteristics of such electrical control signals to provide the radiation which leaves the modulator of phase 11 a form of desired spectral envelope, with the spectral width Δν ₁ . Generator 11c can be activated by transmit controller 50 selectively to produce pulses 1, or can be activated permanently. The reference 20 designates another laser source, that is to say another laser oscillator, which is capable of producing another continuous laser beam, which has the optical frequency ν ₂ , directly with the spectral width Δν ₂ . For example, the laser source 20 can be of the fiber laser type. Indeed, the spectral width value Δν ₂ being low for the invention, it can be supplied directly, or intrinsically, by the laser source 20, that is to say without using any additional component which is specifically dedicated to producing this spectral width value. The two laser sources 10 and 20 constitute the laser source assembly as designated in the general part of this description. The two rays which come respectively from the phase modulator 11 and from the laser source 20 are then injected into two inputs of an optical switch 30, which is denoted COMMUTATOR. This may be a 2 x 1 optical switch which is controlled by transmit controller 50 to output received radiation on either of its two inputs, during the first and/or second intervals of time, and according to the sequence of alternation which is desired between the radiation pulses at the optical frequency ν ₁ and those at the optical frequency ν ₂ . Alternatively, the optical switch 30 can be replaced by a fiber-optic Y coupler, for example with an intensity ratio of 50/50 and possibly with polarization maintenance, or even replaced by a polarization coupler, for example of the cubic splitter type. polarization. Then the radiation as it comes from the optical switch 30 is introduced into an intensity modulator 31, which is denoted MOD. INT. and controlled by the emission controller 50 so that the radiation which is finally emitted in the direction of the measurement zone has the instantaneous power value P ₁ during the first time intervals of durations Δt ₁ where the optical frequency is closer to the value ν ₁ , and has the instantaneous power value P ₂ during the second time intervals of durations Δt ₂ where the optical frequency is closer to the value ν ₂ . The intensity modulator 31 can be of the electro-optical or electro-acoustic type, or with a semiconductor optical amplifier. In known manner, such an intensity modulator can incorporate an internal controller, or be associated with an external controller which is inserted between this intensity modulator and the emission controller 50. The radiation which comes from the intensity modulator 31 is then transmitted to an optical amplification assembly 32, or to an optical amplification chain 32, denoted AMPL., in order to actually produce the transmission optical power values P ₁ and P ₂ . Finally, the radiation that comes from the optical amplification assembly 32 is transmitted to the measurement zone by an output optic 33 of the transmission channel of the LIDAR system, denoted OPT.

Several alternative architectures of LIDAR systems can be derived from that of , each time using at least one of the following equivalence principles, applied to the transmission channel architecture of :
- if the laser source 10 is of a type capable of producing a laser beam at the optical frequency ν ₁ directly with the spectral width value Δν ₁ , such as the laser source 20 of for the spectral width value Δν ₂ , then the phase modulator 11 can be eliminated so that the laser beam from the laser source 10 can be transmitted directly to the optical switch 30, as for the laser source 20. The configuration of is thus obtained;
- when the laser source 10 is of a tunable type, the electrical control signal which is used to confer the spectral width Δν ₁ on the radiation which is emitted around the optical frequency ν ₁ , can be applied directly to a control input of the tunable laser source 10. Such a mode of achieving the desired spectral width is sometimes referred to as internal phase modulation, as opposed to using a phase modulator which is external to the laser source as shown in . Internally modulated laser sources are, for example, laser diodes for which the electric injection current in the gain zone can be modulated with a low modulation amplitude, or DBR diodes, for diodes with distributed Bragg reflector or “Distributed Bragg Reflector diodes” in English, for which the injection into the phase, grating or semiconductor optical amplification areas can be modulated. Alternatively or in combination, this method of obtaining the desired spectral width, internally to the laser source, can also be applied to the laser source 20, when the latter is itself of the tunable type, to obtain the spectral width Δν ₂ . The configuration of is thus obtained, where the references 11c and 21c designate the modulation signal generators which are connected to the respective control inputs of the tunable laser sources 10 and 20;
- two separate external phase modulators can be used simultaneously, one between the laser source 10 and the optical switch 30 to provide the spectral width Δν ₁ to the radiation which is emitted at the optical frequency ν ₁ , as in the case of , and the other between the laser source 20 and the optical switch 30 to provide the spectral width Δν ₂ to the radiation which is emitted at the optical frequency ν ₂ . The configuration of is thus obtained, where the references 11 and 21 designate the two external phase modulators which are respectively associated with the laser sources 10 and 20, and the references 11c and 21c designate the phase modulation signal generators which are connected to the control inputs respective of these phase modulators 11 and 21; And
- a single phase modulator can be used to be effective for the two radiations which are produced separately at the optical frequencies ν ₁ and ν ₂ by the laser sources 10 and 20. In this case, the laser beams of the two laser sources 10 and 20 are transmitted directly to the inputs of the optical switch 30, and the single phase modulator is located between the output of the optical switch 30 and the input of the intensity modulator 31. This single phase modulator can then be controlled from one of the previously described ways, to produce the spectral width Δν ₁ during the first time intervals where the optical switch 30 transmits radiation which has the optical frequency ν ₁ , and to produce the spectral width Δν ₂ during the second time intervals where the switch optical 30 transmits radiation which has the optical frequency ν ₂ . The configuration of is thus obtained, where the reference 34 designates the external phase modulator which is common to the two radiations of the optical frequencies ν ₁ and ν ₂ , and the reference 34c designates the phase modulation signal generator which is connected to the input of control of this phase modulator 34.

The embodiment of can be obtained from that of by carrying out intensity modulations of the radiation located around the optical frequencies ν ₁ and ν ₂ upstream of the meeting of the separate channels for generating these radiations. The way of generation of the radiation which is located around the optical frequency ν ₁ is identical to that of , by adding an intensity modulator 12 thereto. Similarly, the path for generating the radiation which is situated around the optical frequency ν ₂ is identical to that of , by adding an intensity modulator 22 thereto. The two intensity modulators 12 and 22 can be controlled by the transmission controller 50 in a way which is correlated in time with the modulation signal which is produced by the generator 11c . In particular, they produce transmission time windows which limit the first time intervals, of individual durations Δt ₁ , and the second time intervals, of individual durations Δt ₂ , to the desired repetition frequency. For such an embodiment, the separate radiation generation paths located around the two optical frequencies ν ₁ and ν ₂ can be joined together using a coupler 35, in the direction of the downstream part of the transmission path, which is common at the two optical frequencies and which comprises the optical amplification assembly 32. The coupler 35 can be a usual Y coupler. Alternatively, it may be a polarization coupler, which is capable of assigning a determined polarization to the radiation which is transmitted during the first time intervals, of individual durations Δt ₁ , and the orthogonal polarization to the radiation which is transmitted during the second intervals of time, of individual durations Δt ₂ . For example, a linear polarization parallel to a fixed direction can be imparted by the polarization coupler 35 to the radiation which is transmitted during the first time intervals of individual durations Δt ₁ , and a linear polarization perpendicular to the fixed direction can be imparted by the polarization coupler 35 to the radiation which is transmitted during the second time intervals of individual durations Δt ₂ .

The embodiment of can be obtained in the same way as that of , but from the embodiment of , instead of that of . By applying the same method, those skilled in the art will be able to deduce still other possible embodiments for the invention, for example by replacing in those of , Or the single intensity modulator by two intensity modulators which are dedicated separately to the radiation located around the two optical frequencies ν ₁ and ν ₂ .

All embodiments of - And - are adapted to produce radiation emission sequences that conform to And .

In contrast to the embodiments of - And - , the phase modulation that is used in embodiments of And , to obtain the spectral widths Δν ₁ and Δν ₂ which are desired separately during the durations of time intervals Δt ₁ and Δt ₂ , is common for the two optical frequencies ν ₁ and ν ₂ . It is then possible to use a single phase modulator, which is then located downstream of the optical switch or of the optical coupler which groups together in the common downstream part of the transmission path the beams coming separately from the two laser sources 10 and 20 A cost reduction can thus be obtained for the LIDAR system.

These embodiments of And are adapted to produce radiation emission sequences that conform to And .

In the embodiment of , the phase modulation and the intensity modulation are both carried out downstream of the meeting of the optical paths of the radiation which come separately from the laser sources 10 and 20. This meeting of the optical paths is carried out by the switch 30, the modulation of phase is produced by the phase modulator 34, and the intensity modulation is produced by the intensity modulator 31. The phase modulation signal generator 34c can still be of one of the types presented above: PRBS generator , RF noise generator or AWG generator. Switch 30, phase modulation signal generator 34c, and intensity modulator 31 can all be synchronously controlled by transmit controller 50.

In the embodiment of , the radiation which comes separately from the laser sources 10 and 20 is modulated in intensity by using two separate modulators, which are designated by the references 12 and 22. The intensity modulated radiation which comes from these can be introduced by a coupler 35 in the common downstream part of the transmission path. The coupler 35 can also be a Y coupler or a polarization coupler as indicated above. The downstream part of the transmission path then comprises the phase modulator 34, the optical amplification assembly 32 and the output optics 33.

shows a first detection channel architecture which can be used in a LIDAR system according to the invention. The reference 40 designates an input optic, denoted OPT., of the detection path. Its function is to collect part of the backscattered radiation that corresponds to the radiation emission sequence produced by the LIDAR system. This collected portion of radiation is directed onto an optical sensor 43, denoted DETECT. OPT., which produces an electrical detection signal whose intensity is a function of the power of the part of the radiation which is detected. The optical sensor 43 can be implemented according to a direct detection mode, or according to a coherent detection mode. A direct detection optical sensor can be made up of a photodiode which is associated with a transimpedance electrical amplifier, for example. An optical sensor with coherent detection, also called heterodyne detection, requires mixing the backscattered radiation which is collected by the input optics 40 with part of the radiation which is produced by the laser source assembly. The electrical output of the optical sensor 43 is connected to the input of an analysis chain 512, denoted ANALYS., which processes the electrical signals delivered by the sensor 43 regardless of the pulse 1 or 2 at which each electrical signal matches.

shows a second detection path architecture which uses two distinct analysis chains which are dedicated separately to radiation pulses 1 and to radiation pulses 2. An electric switch 44, denoted COMM. ELEC., then directs the electrical detection signal towards two distinct analysis chains 51 and 52, denoted ANALYS. 1 and ANALYSIS. 2, depending on which parts of the electrical detection signal correspond separately to first or second time intervals in the emission sequence of the radiation. For this, a synchronization of the operation of the electrical switch 44 can be controlled by the transmission controller 50. Thus, the analysis chain 51 can be dedicated to the first time intervals of individual durations Δt ₁ , and designed to determine the residual absorption of the measurement zone outside the spectral absorption band of the chemical compound whose quantity is to be determined, and to determine the distance of the obstacle which is located in the background of this measurement zone . Independently, the analysis chain 52 can be dedicated to the second time intervals of individual durations Δt ₂ , and designed to determine the absorption of the measurement zone in the absorption spectral band of the chemical compound. A calculation module, not shown, produces an evaluation of the quantity of the chemical compound according to the absorption levels determined by the two analysis chains 51 and 52, and the distance away from the obstacle of background.

shows a third possible detection channel architecture, in which two optical sensors 41 and 42, denoted DETECT. OPT. 1 and DETECT. OPT. 2, separately produce electrical detection signals which are transmitted separately to the analysis chains 51 and 52. An advantage of this third architecture is the possibility of using optical sensors 41 and 42 whose sensitivity levels are different, and adapted separately to the respective instantaneous power values of the useful parts of the backscattered radiation: P ₁ in the first time intervals of individual durations Δt ₁ for the sensor 51, and P ₂ in the second time intervals of individual durations Δt ₂ for the sensor 52 In this case, the backscattered radiation which is collected by the input optics 40 is directed onto the sensor 41 or onto the sensor 42 by an optical switch 45. The operation of this optical switch 45 is controlled by the optical controller. transmission 50. For this third detection channel architecture, the coupler 35 of the transmission channel can be of the Y coupler type with a ratio of 50/50.

The three architectures of - for the detection channel are each compatible with the architecture variants of - , - And - for the transmission channel.

Finally, shows a fourth possible detection channel architecture, in which the optical switch 45 of is replaced by a polarization separator 46, denoted SEP. POLAR., which is based on linear polarizations of radiation. This fourth architecture for the detection channel is compatible with the embodiments for the transmission channel where the coupler 35 is of the polarization coupler type, as described above.

In general, the optical sensor or at least one of the optical sensors used in the detection channel has a sufficiently short reaction time to make it possible to estimate the distance at which the background obstacle is moved from of the radiation emitted during the durations Δt ₁ .

In addition, each analysis chain of the detection path can be connected at the output to a calculation unit (not shown), which is suitable for supplying from signals produced by the or both chain(s) of analysis, estimates of the distance away from the background obstacle and of the quantity of the chemical compound which is present in the measurement zone, integrated in the path of the pulses. Such a calculation unit can optionally be integrated into the LIDAR system.

It is understood that the invention can be reproduced by modifying secondary aspects of the embodiments which have been described in detail above, while retaining at least some of the advantages cited. In particular, optical components with equivalent functions can be used instead of those which have been mentioned. In addition, the following modifications are mentioned by way of example as alternatives available to those skilled in the art without inventive step:
- the same laser source switchable in optical frequency can be used to produce the pulses at the optical frequencies ν ₁ and ν ₂ ;
- separate optical amplifiers can be used for the pulses at the optical frequency ν ₁ and those at the optical frequency ν ₂ ;
- the interleaving of the pulses at the optical frequency ν ₁ with those at the optical frequency ν ₂ to produce the radiation emission sequence can be carried out before or after the application to each pulse of the phase modulation and/or of intensity modulation; And
- the interleaving of the pulses at the optical frequency ν ₁ with those at the optical frequency ν ₂ to produce the radiation emission sequence can be carried out before or after the optical amplification of the pulses.
Finally, all the numerical values that have been cited have been cited for illustration purposes only, and can be changed depending on the chemical compound whose quantity is to be determined.

Claims (13)

LIDAR system, suitable for carrying out differential absorption measurements between two distinct optical frequencies (ν ₁ , ν ₂ ), and distance measurements of an obstacle which is present in the background of a measurement zone where absorption occurs, the LIDAR system comprising:
- A laser source assembly, capable of producing radiation at either of the two optical frequencies (ν ₁ , ν ₂ );
- Intensity modulation means, adapted to apply to each radiation a form of pulse envelope, including a pulse duration and a pulse optical power value (P ₁ , P ₂ ); And
- a transmission controller (50), suitable for controlling the intensity modulation means.
the LIDAR system being adapted so that, during operation of the LIDAR system, said LIDAR system emits in a target direction in which a measurement of differential absorption and a measurement of the distance away from the obstacle of background are to be produced, a radiation emission sequence which includes:
- first time intervals during which the radiation emission is located spectrally at a first (ν ₁ ) of the two optical frequencies, with a first spectral width (Δν ₁ ), a first pulse duration (Δt ₁ ) and a first pulse optical power value (P ₁ ); And
- second time intervals during which the emission of radiation is located spectrally at one second (ν ₂ ) from the two optical frequencies, with a second spectral width (Δν ₂ ), a second pulse duration (Δt ₂ ) and a second pulse optical power value (P ₂ ),
the first (Δν ₁ ) and second (Δν ₂ ) spectral widths being such that the emission of radiation during the first time intervals and during the second time intervals correspond to spectral intervals which are disjoint, the first spectral width being greater at the second spectral width,
the first pulse optical power value (P ₁ ) is greater than the second pulse optical power value (P ₂ ), and
the first pulse duration (Δt ₁ ) being shorter than the second pulse duration (Δt ₂ ),
the LIDAR system further comprising a detection channel which is adapted to independently detect, process and analyze backscattered radiation which corresponds to the first optical frequency (ν ₁ ) and to the second optical frequency (ν ₂ ), and which respectively correspond to transmissions during the first and second time intervals. A LIDAR system according to claim 1, further comprising:
- spectral broadening means, arranged to modify a spectral width of at least one of the radiation produced by the laser source assembly (10, 20). LIDAR system according to claim 1 or 2, according to which the emission controller (50) is adapted to control the laser source assembly, the intensity modulation means and optionally the spectral broadening means so that, in the radiation emission sequence, the first (ν ₁ ) of the two optical frequencies is associated exclusively with the first spectral width (Δν ₁ ), the first pulse duration (Δt ₁ ) and the first optical power value d pulse (P ₁ ), excluding the second spectral width (Δν ₂ ), the second pulse duration (Δt ₂ ) and the second pulse optical power value (P ₂ ), within of first radiation pulses (1), and the second (ν ₂ ) of the two optical frequencies is associated exclusively with the second spectral width, the second pulse duration and the second pulse optical power value, with the exclusion of the first spectral width, the first pulse duration on and the first pulse optical power value, within second radiation pulses (2) which are separated from said first pulses. LIDAR system according to claims 2 and 3, wherein
the laser source assembly includes a first laser oscillator (10) which is adapted to produce the radiation at the first optical frequency (ν ₁ ), and a second laser oscillator (20) which is adapted to produce the radiation at the second optical frequency (ν ₂ ) with the second spectral width (Δν ₂ ),
the spectral broadening means comprises a phase modulator (11) which is disposed in a path of the radiation produced by the first laser oscillator (10), and which is controlled by the emission controller (50) to provide the first width spectral (Δν ₁ ) to said laser radiation produced by the first laser oscillator; And
the LIDAR system further comprises an optical switch (30) which is controlled by the emission controller (50) to transmit either the radiation coming from the phase modulator (11) or the radiation produced by the second laser oscillator (20), to a downstream portion of the optical transmission path which is common to the said radiation coming from the phase modulator and produced by the second laser oscillator, the said downstream portion of the optical transmission path comprising the intensity modulation means (31). LIDAR system according to claims 2 and 3, wherein
the laser source assembly includes a first laser oscillator (10) which is adapted to produce the radiation at the first optical frequency (ν ₁ ), and a second laser oscillator (20) which is adapted to produce the radiation at the second optical frequency (ν ₂ ) with the second spectral width (Δν ₂ ),
the spectral broadening means comprises a phase modulator (11) which is disposed in a path of the radiation produced by the first laser oscillator (10), and which is controlled by the emission controller (50) to provide the first width spectral (Δν ₁ ) to said radiation produced by the first laser oscillator,
the intensity modulation means comprises a first intensity modulator (12) which is arranged in a path of the radiation coming from the phase modulator (11), and which is controlled by the transmission controller (50) to be effective on said radiation from the phase modulator, and include a second intensity modulator (22) which is disposed in a path of the radiation produced by the second laser oscillator (20), and which is controlled by the emission controller to be effective on said radiation produced by the second laser oscillator,
and the LIDAR system further comprises an optical coupler (35) adapted to transmit the radiation originating from the first (12) and second (22) intensity modulators to a downstream portion of the transmission optical path which is common to the said radiation originating from the first and second intensity modulators. LIDAR system according to claim 2, wherein the emission controller (50) is adapted to control the laser source assembly, the spectral broadening means and the intensity modulation means so that the emission sequence of radiation comprises a succession of radiation pulses which are located spectrally either at the first (ν ₁ ) of the two optical frequencies, or at the second (ν ₂ ) of the two optical frequencies, all the radiation pulses having the same shape of envelope which comprises, for the first optical frequency as well as for the second optical frequency, the first duration (Δt ₁ ) during which the radiation emission has the first spectral width (Δν ₁ ) and the first value of pulse optical power ( P ₁ ), and a second duration (Δt ₂ ) during which the radiation emission has the second spectral width (Δν ₂ ) and the second pulse optical power value (P ₂ ), the first duration being pl us shorter than the second duration, and being earlier or later than said second duration in each radiation pulse. LIDAR system according to any one of the preceding claims, made using fiber optic technology. LIDAR system according to any one of the preceding claims, comprising polarization means (35) adapted so that the radiation which is emitted in the direction of the measurement zone by the said LIDAR system has polarizations which are orthogonal, in particular circular polarizations which are opposite, when said radiations are emitted during the first time intervals or the second time intervals,
and the detection path comprises a polarization splitter (46) which is arranged to transmit the backscattered radiation, as a function of polarizations of said backscattered radiation, either to a first detector (41) which is sensitive in a first spectral interval including the first frequency optical (ν ₁ ) combined with the first spectral width (Δν ₁ ), or towards a second detector (42) which is sensitive in a second spectral interval including the second optical frequency (ν ₂ ) combined with the second spectral width (Δν ₂ ). A method of measuring a quantity of a chemical compound which is present in a target direction, according to which:
- a LIDAR system which is in accordance with any one of the preceding claims, is selected such that the chemical compound has an absorptive power value which is lower at the first optical frequency (ν ₁ ) than at the second frequency optical (ν ₂ );
- the LIDAR system is oriented along the target direction to emit radiation according to the radiation emission sequence towards a measurement zone which is likely to contain the chemical compound, and an operation of the LIDAR system is triggered;
- the distance away from the obstacle which is present in the background of the measurement zone is estimated from the backscattered radiation which is relative to the first optical frequency (ν ₁ ) and which corresponds to emissions during the first time intervals; And
- the quantity of the chemical compound which is contained in the measurement zone, integrated on a path of the pulses between the LIDAR system and the background obstacle, is estimated from intensity values which are separately related to the radiation backscattered at the first optical frequency (ν ₁ ) and at the second optical frequency (ν ₂ ), corresponding respectively to the first and second time intervals in the radiation emission sequence, said backscattered radiation having been detected by the detection channel of the LIDAR system. Method according to Claim 9, in which the distance away from the obstacle which is present in the background of the measurement zone is estimated from a time of flight which is measured for the backscattered radiation relating to the first optical frequency (ν ₁ ) ( λ _OFF ) . Method according to Claim 10, in which the distance away from the obstacle which is present in the background of the measurement zone, as estimated from the backscattered radiation which is relative to the first optical frequency (ν ₁ ) ( λ _OFF ) , is used to estimate the quantity of the chemical compound which is contained in the measurement zone, in combination with the intensity values which are relative separately to the backscattered and detected radiation at the first optical frequency and at the second optical frequency (ν ₂ ), and corresponding respectively to the emissions during the first and second time intervals. A method according to any of claims 9 to 11, wherein the chemical compound is one of carbon dioxide, methane, nitrous oxide and water. A method according to any one of claims 9 to 12, wherein the LIDAR system is installed on the Earth's surface and directed to measure the amount of chemical compound which is present between said LIDAR system and the obstacle,
or the LIDAR system is on board an aircraft in flight and directed towards a geographical area on the surface of the Earth, to measure the distance of the distance from the surface of the Earth in said geographical area with respect to the LIDAR system, and to measure the amount of the chemical compound that is present between said LIDAR system and the Earth's surface in said geographical area,
or the LIDAR system is on board a satellite in orbit around the Earth and directed towards a geographical area on the surface of the Earth, to measure the distance of the distance from the surface of the Earth in said geographical area with respect to to the LIDAR system, and to measure the quantity of the chemical compound which is present between said LIDAR system and the surface of the Earth in said geographical area.